Add enhanced default bin/dev

Fixes rails#52459

The current default `bin/dev` execs the Rails server directly and
doesn't support Procfiles. Therefore, gems like `jsbundling-rails`,
`tailwindcss-rails`, and soon `solid_queue` need to overwrite the
`bin/dev` script with their own version.

With this new version, gems can skip overwriting the `bin/dev` script
and add to the Procfile instead.

It spawns Foreman in the background to allow breakpoints in the Rails
process without Foreman eating the inputs or interleaving its output.

railties/CHANGELOG.md

@@ -1,3 +1,11 @@
+*   Enhanced `bin/dev` Script
+
+    Previously, `bin/dev` would only start the Rails server. Gems requiring multiple processes override the `bin/dev` script to start Foreman instead, which prevents interaction with the Rails server console (debugger).
+
+    This new `bin/dev` script starts Foreman in the background if a `Procfile.dev` is present and starts Rails in the foreground to allow debugging with breakpoints.
+
+    *Joé Dupuis*
+
 ## Rails 8.0.0.beta1 (September 26, 2024) ##
 
 *   Exit `rails g` with code 1 if generator could not be found.

railties/lib/rails/generators/rails/app/templates/bin/dev.tt

@@ -1 +1,36 @@
-exec "./bin/rails", "server", *ARGV
+ENV["PORT"] ||= "3000"
+VERBOSE = ARGV.include?("--verbose") || ARGV.include?("-v") || ENV["VERBOSE"]
+RAILS_ARGS = ARGV - [ "--verbose", "-v" ]
+PROCFILE = "Procfile.dev"
+Process.exec(*%w[bundle exec rails server], *RAILS_ARGS) unless File.exist?(PROCFILE)
+
+Signal.trap("INT") { }
+
+if system(*%w[gem list --no-installed --exact --silent foreman])
+  puts "Installing foreman..."
+  system(*%w[gem install foreman])
+end
+
+begin
+  in_r, in_w = IO.pipe
+  in_w.close
+  foreman_pid = Process.spawn(
+    *%w[foreman start -f Procfile.dev -m all=1,web=0 --env /dev/null],
+    in: in_r,
+    out: (VERBOSE ? STDOUT : "/dev/null"),
+    err: (VERBOSE ? STDOUT : "/dev/null"),
+    pgroup: true,
+  )
+  foreman_pgid = Process.getpgid(foreman_pid)
+  pid = Process.spawn(*%w[bundle exec rails server], *RAILS_ARGS)
+
+  Process.wait(pid)
+  pid = nil
+ensure
+  Process.kill("INT", -foreman_pgid)
+  Process.wait(-foreman_pgid)
+  begin
+    Process.kill("KILL", pid) if pid
+  rescue Errno::ESRCH
+  end
+end